How To Diagnose a Blown Head Gasket

It is important to properly maintain the head gasket of an engine, since it performs a critical function of sealing all coolant passages, combustion chambers and oil passages in between the block and the head. A head gasket has to be a leak-free seal from the time it is installed up to the time the engine can no longer be used.

A head gasket usually fails due to overheating. Overheating occurs when a coolant leaks, or when a fan or the thermostat fails. The causes of head gasket failures can be grouped into:

1.    Flaws in the design of the gasket or engine like:

  • a hard to seal engine
  • bimetal engine that creates thermal stress
  • or weakness in the gasket design

2.    Errors in the installation using:

  • a dirty head or unclean block
  • flat, smooth or a wrong finish of the gasket surface
  • a wrong bolt procedure, sequence, or specifications of the torque when the bolts of the head gasket were tightened
  • or reusing damaged, dirty or stretched bolts
  • a sealer on gaskets that do not need a sealer

3.    Overstressed gasket conditions in operation causing it to fail due to:

  • preignition
  • detonation
  • overheating

The first step to prevent a gasket from repeatedly failing is to find out the cause of the first head gasket failure. There are instances when the cause is easy to diagnose or is obvious, but there are times you need to look into various changes in the gasket condition to determine the cause. A head gasket usually fails when the lubricating oil or coolant intermixes or one of the fluids enters the cylinders.

When diagnosing a blown head gasket, look into the following:

  1. Check the cylinder bores.  Head gasket failure is commonly detected in the cylinder. When the gasket fails, the oil or coolant goes to the combustion chamber. Check if you can smell a 'sweet' odor of ethylene-glycol or propylene-glycol coolant from the exhaust of the vehicle, or the coolant smelling like exhaust.
  2. Make a visual inspection of the piston top and spark plugs for the formation of carbon soot coming from the cylinder wall interior. If the piston top and spark plugs are clean, the coolant has entered the cylinders and removed the carbon soot.
  3. Look for carbon deposits on the spark plug's insulator and electrodes. If there are deposits, the spark plugs foul and that means that oil has entered the cylinders. Carbon found on insulators also causes a spark plug not to spark and causes the electrical circuit to have lower resistance created by the conductive carbon film on top of the insulator.
  4. Check to see if there is oil present in the coolant of the engine. This is a sign that your head gasket may have blown. A coolant is recognized by the color of the liquid. If you find oil under the engine, it may just be water that blew over piston rings and condensed from combustion.
  5. Using a micrometer, check if the head gasket that failed is crushed or thinner in some areas. If the area surrounding the combustion chamber has a cracked armor, this may have caused a burn-through. The head gasket may have failed due to preignition or detonation.

To prevent a blown head gasket, make sure to use the right kind of coolant with the right mixture that matches your engine. Regularly keep your cooling system clean to prevent overheating, and if needed use "high temperature" gaskets to avoid detonation.


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